DC-DC converters typically utilize some type of buck converter which has an inductive element and capacitive element arranged to provide a tank circuit having an input and an output. This tank circuit receives a switching input on the input side, this input side being switched between a positive DC voltage and ground. In one configuration, an asynchronous configuration, an input node has a diode disposed between the input and ground and a first switching transistor switch disposed between the input and the positive DC voltage. The switching rate of the first switching transistor defines what the output voltage will be. In another mode, a synchronous mode, the diode is replaced by a second switching transistor. This second switching transistor is switched open whenever the first switching transistor is switched closed and the second switching transistor switched closed whenever the first switching transistor is switched open. In this manner, the input to the tank circuit is connected to either the positive input voltage or to ground.
In the case of the asynchronous switching operation, regulation is maintained relatively well at low currents. However, at high currents, the efficiency is degraded due to power lost in the flyback diode. With respect to the synchronous switching operation, this has some drawbacks for low current operation in that the operation of the switching FET connected to ground can consume more power than the flyback diode. The synchronous operation is better for high current applications whereas the asynchronous operation is much more useful for the low current operations.
With respect to efficiency of operation, there have been provided switching regulators that vary the type of modulation in order to select the most efficient operation for a given current level. For example, one switching type regulator may operate under pulse width modulation and one may operate under pulse frequency modulation. These two types of modulation have different types of efficiencies depending upon the current; i.e., one topology operates more efficiently at low currents as compared to the other, which operates more efficiently at high currents. By selecting between the two, a better efficiency rating can be obtained for the overall switching regulator as a function of current.